Network Working Group W. Kumari
Internet-Draft Google
Intended status: Informational R. Arends
Expires: May 3, 2017 Nominet
S. Woolf
D. Migault
Orange
October 30, 2016
Highly Automated Method for Maintaining Expiring Records
draft-wkumari-dnsop-hammer-02
Abstract
This document describes a simple DNS cache optimization which keeps
the most popular records in the DNS cache: Highly Automated Method
for Maintaining Expiring Records (HAMMER). The principle is that
records in the cache are fetched, that is to say resolved before
their TTL expires and the record is flushed from the cache. By
fetching Records before they are being queried by an end user, HAMMER
is expected to improve the quality of experience of the end users as
well as to optimize the resources involved in large DNSSEC resolving
platforms.
Status of This Memo
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This Internet-Draft will expire on May 3, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Improving browsing Quality of Experience by reducing
response time . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Optimize the resources involved in large DNSSEC resolving
platforms . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
5. Known implementations . . . . . . . . . . . . . . . . . . . . 5
5.1. Unbound (NLNet Labs) . . . . . . . . . . . . . . . . . . 5
5.2. OpenDNS . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. ISC BIND . . . . . . . . . . . . . . . . . . . . . . . . 6
6. An example / reference implementation . . . . . . . . . . . 6
6.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
A recursive DNS resolver may cache a Resource Record (RR) for, at
most, the Time To Live (TTL) associated with that record. While the
TTL is greater than zero, the resolver may respond to queries from
its cache; but once the TTL has reached zero, the resolver flushes
the RR. When the resolver gets another query for that resource, it
needs to initiate a new query. This is then cached and returned to
the querying client. This document discusses an optimization (Highly
Automated Method for Maintaining Expiring Records -- (HAMMER), also
known as "prefetch") to help keep popular responses in the cache, by
fetching new responses before the TTL expires. This behavior is
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triggered by an incoming query that arrives only shortly before the
cache entry was due to expire.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Terminology
HAMMER resolver: A DNS resolver that implements HAMMER mechanism.
HAMMER FQDN: A FQDN that is a candidate for the HAMMER process.
HAMMER TIME: TTL Time to consider before triggering the HAMMER
mechanism.
3. Motivations
When a recursive resolver responds to a client, it either responds
from cache, or it initiates an iterative query to resolve the answer,
caches the answer and then responds with that answer.
3.1. Improving browsing Quality of Experience by reducing response time
Any end user querying a fetched FQDN will get the response from the
cache of the resolver. This provides faster responses, thus
improving the end user experience for browsing and other
applications/activities.
Popular FQDNs are highly queried, and end users have high
expectations in terms of application responsiveness for these FQDNs.
With regular DNS rules, once the FQDN has been flushed from the
cache, it waits for the next end user to request the FQDN before
initiating a resolution for this given FQDN with iterative queries.
This results in at least one end user waiting for this resolution to
be performed over the Internet before the response is sent to them.
This may provide a poor user experience since DNS response times over
the Internet are unpredictable at best and it provides a response
time longer then usual.
In some cases, not only the first end user querying that FQDN may be
impacted, but also other end users that request the FQDN between the
time the FQDN TTL expires and the time the cache is again filled. In
this case, the result is impact on multiple end users and possible
unnecessary load on the platform. Note that this load is increased
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by the use of DNSSEC since DNSSEC may involve additional resolutions,
larger payloads, and signature checks.
3.2. Optimize the resources involved in large DNSSEC resolving
platforms
Large resolving platforms are often composed of a set of independent
resolving nodes. The traffic is usually load balanced based on the
query source IP addresses. This results in most popular FQDNs being
resolved independently by all nodes. This increases the number of
end users who may experience unnecessary latency. Also, when DNSSEC
is used, all nodes independently perform signature check operations,
possibly resulting in high loads on the authoritative server.
The challenge these large DNSSEC resolving platforms have to overcome
is to provide a uniform distribution of the nodes given that end user
and FQDNs do not have a uniform distribution of the resources. More
specifically, FQDNs and end users usually present Zipf popularity
distributions, which means that most of the traffic is performed by a
small set of end users and by a small set of FQDNs.
DNS and large resolving DNS platforms have resulted in uniformly
balanced traffic among the nodes. In fact the resolving traffic on
the Internet interface was rather small (at least in term of CPU)
compared to traffic received from the end users. DNSSEC changed
this, as CPU are involved in performing signature checks. One way to
reduce the number of DNSSEC resolutions is to fetch the nodes with
the most popular FQDNs. This avoids parallel resolutions and overall
reduces cost, because signature checks are not performed, while
benefiting from the already existing load balancing architecture.
This architecture takes advantage of the Zipf distribution of the
FQDNs' popularity. In fact, a few number of FQDNs can be cached (a
few thousands) to address most of the traffic (up to 70%).
Note that to perform a single resolution for the global platform,
nodes may be configured as forwarders for the most popular FQDNs
4. Overview of Operation
When an incoming query is received, and the result is in the cache,
the query is answered from the cache. If the remaining TTL of the
record is below some threshold, the recursive server will also
initiate a cache fill operation in the background to refresh the
cache entry.
The fact that the behavior is triggered by an incoming query (and not
by periodically scanning the cache and refreshing all entries that
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are about to expire) allows unpopular names to age out of the cache
naturally, while keeping popular entries in the cache.
5. Known implementations
[Ed: Well, this is kinda embarrassing. This idea occurred to us one
day while sitting around a pool in New Hampshire. It then took a
while before I wrote it down, mostly because I *really* wanted to get
"Stop! Hammer Time!" into a draft. Anyway, we presented it in
Berlin, and Wouter Wijngaards stood up and mentioned that Unbound
already does this (they use a percentage of TTL, instead of a number
of seconds). Then we heard from OpenDNS that they *also* implement
something similar. Then we had a number of discussions, then got
sidetracked into other things. Anyway, BIND as of 9.10, around Feb
2014 now implements something like this
(https://deepthought.isc.org/article/AA-01122/0/Early-refresh-of-
cache-records-cache-prefetch-in-BIND-9.10.html), and enables it by
default. Unfortunately, while BIND uses the times based approach,
they named their parameters "trigger" and "eligibility" - and
shouting "Eligibility! Trigger time!" simply isn't funny (unless you
have a very odd sense of humor... So, we are now documenting
implementations that existed before this was published and an
impl,entation that we think was based on this. We think that this
has value to the community. I'm also leaving in the HAMMER TIME bit,
because it makes me giggle. This below section should be filled out
with more detail, in collaboration with the implementors, but this is
being written *just* before the draft cutoff.].
A number of recursive resolvers implement techniques similar to the
techniques described in this document. This section documents some
of these and tradeoffs they make in picking their techniques.
5.1. Unbound (NLNet Labs)
The Unbound validating, recursive, and caching DNS resolver
implements a HAMMER type feature, called "prefetch". This feature
can be enabled or disabled though the configuration option "prefetch:
<yes or no>". When enabled, Unbound will fetch expiring records when
their remaining TTL is less than 10% of their original TTL.
[Ed: Unbound's "prefetch" function was developed independently,
before this draft was written. The authors were unaware of it when
writing the document.]
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5.2. OpenDNS
The public DNS resolver, OpenDNS implements a prefetch like solution.
[Ed: Will work with OpenDNS to get more details.]
5.3. ISC BIND
As of version 9.10, Internet Systems Consortium's BIND implements the
HAMMER functionality. This feature is enabled by default.
The functionality is configured using the "prefetch" options
statement, with two parameters:
Trigger This is equivalent to the HAMMER_TIME parameter described
below.
Eligibility This is equivalent to the STOP parameter described
below.
6. An example / reference implementation
When a recursive resolver that implements HAMMER receives a query for
information that it has in the cache, it responds from the cache.
If the queried FQDN is a HAMMER FQDN, the HAMMER resolver compares
the TTL value to the HAMMER TIME, as well as if the FQDN has already
been fetched.
If the HAMMER FQDN has already been fetched or provisioned) then
nothing is done.
If the HAMMER FQDN has not yet been fetched and the TTL is less than
the HAMMER_TIME, the HAMMER resolver starts a resolution for the
queried FQDN in order to fill the cache, just as if the TTL had
expired. During this cache fill operation the resolver continues to
respond from cache (until the TTL expires). When the cache fill
query completes, the new response replaces the existing cached
information. This ensures the cache has fresh data for subsequent
queries.
Since the cache fill query is initiated before the existing cached
entry expires (and is flushed), responses will come from the cache
more often. This decreases the client resolution latency and
improves the user experience.
The cache fill resolution is triggered by an incoming query (and only
if that query arrives shortly before the record would expire anyway).
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This effectively keeps the most popular data uniformly queried in the
cache, without having to maintain counters in the cache or
proactively resolve responses that are not likely to be needed as
often. This is purely an implementation optimization - resolvers
always have the option to cache records for less than the TTL (for
example, when running low on cache space, etc), this simply triggers
a refresh of the RR before it expires.
Note that non-uniformly queried FQDNs may be popular and may not
benefit from the HAMMER mechanism. For example, an FQDNs MAY be
heavily queried the first 10 minutes of every hour with a 30 minute
TTL. In that case DNS queries are not expected to come between TTL -
HAMMER_TIME and TTL.
HAMMER FQDNs with small TTL may generate a cache fill process even
though they are not so popular. Suppose an end user is setting a
specific session which requires multiple DNS resolutions on a given
FQDN. These resolutions are necessary for a short period of time,
i.e. the necessary time to establish the session. If these FQDNs
have been set with a small TTL - in the order of the time session
establishment - the multiple queries to a HAMMER resolver may trigger
an unnecessary resolution. As a result HAMMER would not scale
thousands of these FQDNs. As a result, if the original TTL of the RR
is less than (or close to HAMMER_TIME), the described method could
cause excessive cache fill queries to occur. In order to prevent
this an additional variable named STOP (described below) is
introduced. If the original TTL of the RR is less than STOP *
HAMMER_TIME then the cache entry should be marked with a "Can't touch
this" flag, and the described method should not be used.
6.1. Variables
These are the mandatory variables:
HAMMER_TIME: is the number of seconds before TTL expiration that a
cache fill query should be initiated. This should be a user
configurable value. A default of 2 seconds is RECOMMENDED.
STOP: should be a user configurable variable. A default of 3 is
recommended.
Implementations may consider additional variables. These are not
mandatory but would address specific use of the HAMMER.
HAMMER_MATCH: should be a user configurable variable. It defines
FQDNs that are expected to implement HAMMER. This rule can be
expressed in different ways. It can be a list of FQDNs, or a
number indicating the number of most popular FQDNs that needs
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to be considered. How HAMMER_MATCH is expressed is
implementation dependent. Implementations can use a list of
FQDNs, others can use a matching rule on the FQDNs, or define
the HAMMER_FQDNs as the X most popular FQDNs.
HAMMER_FORWARDER: should be a user configurable variable. It is
optional and designates the DNS server the resolver forwards
the request to.
7. IANA Considerations
This document makes no request of the IANA.
8. Security Considerations
This technique leverages existing protocols, and should not introduce
any new risks, other than a slight increase in traffic.
By initiating cache fill entries before the existing RR has expired
this technique will slightly increase the number of queries seen by
authoritative servers. This increase will be inversely proportional
to the average TTL of the records that they serve.
It is unlikely, but possible, that this increase could cause a denial
of service condition.
9. Acknowledgements
The authors wish to thank Tony Finch and MC Hammer. We also wish to
thank Brian Somers and Wouter Wijngaards for telling us that they
already do this :-) (They should probably be co-authors, but I left
this too close to the draft cutoff time to confirm with them that
they are willing to have their names on this).
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
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[I-D.ietf-sidr-iana-objects]
Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
progress), May 2011.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From -01 to -02:
o Readbility / cleanup.
o Tried to make it more clear that most implementations now support
this (although they call it "prefetch" )
From -00 to 01:
o Fairly large rewrite.
o Added text on the fact that there are implmentations that do this.
o Added the "prefetch" name, cleaned up some readability.
o Daniel's test (Section 3.2) added.
From -template to -00.
o Wrote some text.
o Changed the name.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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Roy Arends
Nominet
Edmund Halley Road
Oxford OX4 4DQ
United Kingdom
Email: roy@nominet.org.uk
Suzanne Woolf
39 Dodge St. #317
Beverly, MA 01915
US
Email: suzworldwide@gmail.com
Daniel Migault
Orange
38 rue du General Leclerc
92794 Issy-les-Moulineaux Cedex 9
France
Phone: +33 1 45 29 60 52
Email: daniel.migaultf@orange.com
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